1. Field of the Invention
The present invention relates to a blood component-separating composition (hereinafter referred to as "blood separation composition") which is employed in a centrifugal separation method to separate serum or plasma from whole blood, wherein the difference in specific gravity between them is utilized.
2. Prior Art
Many kinds of blood separation compositions which have been proposed are useful in the centrifugal method of blood separation. Those prior art separation compositions contain as their main ingredient a gel-like material such as silicone oil, a chlorinated polybutene, an acrylic polymer or a copolymer of an .alpha.-olefin and a diester of maleic acid. Typical additives blended with the main ingredient are a thixotropic agent for enhancing important properties of the gel-like material; and an inorganic substance. The thixotropic agent causes the gel-like material not to flow within the blood collection tubes but to stay on the bottom of the tubes while being transported. When centrifugal force is applied to the collection tubes filled with blood, the gel-like material moves upwards and forms a partition barrier between the serum (or plasma) and the clot. The thixotropic agent also enhances the strength and stability of the partition barrier. On the other hand, an inorganic substance such as titanium dioxide and calcium carbonate is blended with the gel-like material so as to adjust its specific gravity.
The molecules of the thixotropic agents in the prior art blood separation compositions generally have functional groups capable of forming hydrogen bonds. The thixotropic agents are thus either inorganic fine powders such as silica and clay, or organic gelling agents.
There are observed many disadvantages inherent in the prior art thixotropic agents, as summarized below. The inorganic powder such as silica has a specific density which is too high compared with that of the gel-like material used as the main ingredient. If the inorganic powder is blended at a content sufficient to realize a required level of viscosity, then the specific gravity of the blood separation means will often rise above a desirable range of 1.035 to 1.060. On the contrary, adjustment of the specific gravity to fall within this range will be accompanied by an undesirably low viscosity. Thus, it has been difficult to meet the requirement of specific gravity concurrently with the requirement of viscosity. It is another problem that, due to the high specific gravity of the inorganic powder, the actual specific gravity of the blood separation means cannot be kept at a constant level but varies among production lots thereof and over the course of time. In addition, poor compatibility of a rich content of inorganic powder with the gel-like material will cause some fractions of the separation composition to be dispersed in the serum phase (or plasma phase), forming an oily substance which floats therein. Such floating particles are likely to clog the nozzles in an automatic analyzer. Further, because of insufficient strength of the partition barrier, perfect centrifugal isolation of the serum (or plasma) from the clot is not ensured. A number of blood cells will remain in the serum phase above the partition, thus impairing the separation accuracy.
In a case wherein the thixotropic agent added to the blood separation composition is an organic gelling agent, for example, a sorbitol-aromatic aldehyde condensation product, a very low content suffices for a desired level of thixotropy. The extremely low content however brings about inevitably a large variation of viscosity between production lots of said separation composition. The use of such a blood separation composition forms an unstable partition barrier. Moreover, the intermoleoular attraction of such an organic gelling agent becomes intensive during the course of time to such a degree as to increase the minimum shear stress for solation of the separation composition. This means poor stability during storage, and the blood separation composition after being stored for a long time will not smoothly rise (or "ascent") within the centrifuged collection tube, thus lowering its separation capability.